Is this RPM range OK?

[Carlyboy]

I've recently played around with the sprockets on my stock CT70K1 and am now running 16/33. It seems to still have power in reserve in 3rd gear pulling from 20mph, 30mph and 34mph, so it doesn't just die in top gear. At 37mph (60kmh) it should now rev at just less than 6200rpm (compared to 7000rpm with the original 15/35 sprockets. I also tried 16/35 which would give 6500prm at 37mph.

I just want to try to keep up with the local traffic as I only ride it locally and occasionaly on the weekend.

My main questions are:-
1) would it be better to rev at 6500rpm's than 6200 at 37mph. i.e. bit more grunt?
2) and is it OK to hold it at these revs for a while without causing any damage?
3) Is it OK to ride at this speed on the standard split rims and Bridgestone Trail Wings ?- the tyres are new.

Not sure what the red line would be on this bike but I would expect somewhere between 8-10,000rpm? - but you wouldn't sit on these revs anyway.

So, I'm just trying to get the optimum balance of power, speed and rpm's and want to make sure that it's safe to ride at 37mph and also without damaging the engine. I know that a rider's weight, terrain, preference all play a part. this is a stock 6v 3 speed semi-auto with a stock 72cc motor. Appreciate any other members' comments and their own experiences.

 

[mrichard]

I just gotta ask why did you change the sprockets on a stock ct70 bike. I think Honda did alot of testing to get them correct??

 

[P.C.]

37 sounds a little slow for a stock 72cc.in a good state of tune they run around 42.

 

[Carlyboy]

When they're manufactured, they are made to please all applications including where they're ridden i.e. off road or on road, the weight of the rider, kids and adults and the terrain i.e. hills, flat etc etc. and try to get the 'average' ratio to suit all types.

e.g. if it was to ridden purely on a track normally one would opt for a higher reving lower gear ratio. These bikes were designed as a road/trail for both applications, as mine is been ridden on the road I found that it reved too high at higher speeds.

I also geared up my stock Z50 as 1st gear especially was like a tractor. These were probably made with kids in mind who were just learning how to ride.So, I'm just trying to get the best ratio to suit my riding application.

I feel that the 16/33 gearing works quite well but is it OK to hold the revs at say 6200rpm for a while?

 

[OLD CT]

1, it would be better to rev 6500 or higherin top gear.2,it wont hurt it,''if'' the engine is in good shape or somewhat fresh.by the sound of your top speed,it's not.3,split rims are ok up to almost 70mph.thats when wheel balancing comes into play.sounds like 25mph zones are your best friend.35 zones better keep your eyes in the mirrors.

 

[69ST]

A stock K1, in sound mechanical condition, is damn near impossible to over-rev in 3rd gear. Between the short stroke and incredible rod/stroke ratio, the expected failure would the stock clutch basket, somewhere north of 11,000rpm...well beyond the rev capability of a stock 3-speed. As long as the carburetor is jetted properly and oil temp stays below 110C, the motor should be able to keep you cruising all day long at 7,000rpm+, no sweat. If you're more accustomed to V8-powered cars, 7500rpm will probably seem very busy.

I agree with most of what's been said about factory stock gearing, it is a compromise. But, not as big of one as you seem to believe. Just from memory, H-models were geared for 4.8mph/1000rpm in top gear, 3-speeds closer to 5.0. A 20% jump, to 6.0mph/1000rpm strikes me as overly optimistic, unless your local roads are flat and winds are calm...or better yet, all downhill with tailwinds. (Most places where I ride seem to be uphill with strong headwinds, but that's another story:21

Bottom line, I believe that you're over-geared, closer to the limit for an 88cc tune. Gearing, at the limit, is determined by torque. Displacement and torque are directly related. Best guess is that your motor will pull up to its torque peak, never reaching peak hp...and with it, full top speed potential. Top speed is "the poor man's dyno", hp = mph. That's only if gearing is optimal for the engine's powerband. Overgearing, effectively, strangles peak hp. If you're happy with the results of 16/33 gearing, then the only downside is increased clutch wear.

As for the safe speed limit for split rims, it's a non-issue. Most 10-inch tires carry a "J" speed rating, 62mph, this includes the OEM Trailwings. With "M-rated (84mph) tires, 80mph+ won't result in wheel-related failures. This assumes sound rims, properly assembled.

 

[Carlyboy]

Many thanks for all of your comments. 37mph isn't my top speed but the speed that I'm looking at riding it at. It's actually 60kmh here in Australia which equates to about 37mph.

I guess i geared my bikes up a bit mainly cause I felt that they were revving high at higher speeds, but as mentioned, I was probably also comparing it to how a car revs.. and was probably also a bit concerned about 'damaging the engine' - sitting on a higher speed and revs for a period of time.

Now that I know that It's OK at 7k or whatever and that the tyres and rims are also OK, I'll probably gear it back down a bit again- maybe just up a bit from stock as they're not riden off road and really only on the flat. The CT70 doesn't need much change from stock but my Z50 could have pulled out tree stumps in 1st if it had the power. So will try and get that optimum balance between acceleration, torque, speed,the terrain, rider's weight and personal preference.

Now that I know that I wont be over revving the engine, as mentioned, it is better on these small capacity bikes to rev higher than to labour it. Of course as a general rule the higher the cc's the higher gearing that a bike can accept as it has the grunt to go up hills etc.

So. I'll probably end up with 15/33 on the CT70K1 (standard is 15/35) and 13/35 (standard is 12/37) on the Z50J1. This should improve the rideability a bit but without labouring the engine.

 

[69ST]

With an assembled crankshaft and one-piece connecting rod, fitted with a needle bearing, low-speed lugging is likely to be more abusive than over-revving. Needle bearings and a press-assembled crank don't need much oiling and are stable to higher rpm than an automotive-style one-piece crank & split-cap rod with plain bearings. However, the minuscule surface area of the bearings doesn't play well with heavy "hits", such as low-rpm lugging. That can brinell the bearings, leading to failure. The good news is that, with a semi-automatic clutch it's virtually impossible to lug the engine. You'll get terminal clutch slip before revs get that low.

It's up to you to figure out the best balance of power, speed & gearing. A good rule of thumb is set maximum usable (in terms of sustained cruising & reasonable acceleration time to reach) speed at 80% of top speed potential. This somewhat arbitrary limit can be stretched a little. Thus, if you want a roadworthy 60kph, you'll want to have enough hp to reach ~70kph. That will leave you some "reserve" power for headwinds & hills.

I've run my daily rider for up to 30 miles at a stretch without dipping below 7000rpm, even briefly. The motor starts to sound & feel "busy" around 7500rpm, but that's just my view. The engine doesn't seem mind 8000rpm, at all. The carb jetting spot-on. The original spark plug shows no signs of distress, after a lot of miles, and oil temp peaks between 98C - 105C on the hottest summer days.